FIG. 7 is a view showing an interference exposure apparatus used for manufacturing a conventional diffraction grating. Referring to FIG. 7, a laser beam 17a is output from a laser beam source 4. The laser beam 17a is divided into laser beams 17b and 17c by a half mirror 5 and the laser beams 17b and 17c are reflected by mirrors 6a and 6b, respectively, and then applied to a resist 2 on a substrate 1.
FIGS. 8(a), 8(b), 8(d), and 8(e) are cross-sectional views showing steps in conventional manufacture of a diffraction grating using an interference exposure method and FIG. 8(c) is a diagram showing a distribution of exposure light intensity.
Referring to FIG. 8(a), a resist 92 is applied to a substrate 91. Referring to FIG. 8(b), the resist 92 is exposed by a two-beam interference exposure method. Referring to FIG. 8(c), the exposure light intensity of a laser beam applied to the resist 92 varies periodically. Referring to FIG. 8(d), when the exposed resist 92 is developed, the resist 92 is patterned and a diffraction grating 93a is formed. Thereafter, referring to FIG. 8(e), the substrate 91 is etched away using the patterned resist 92 as a mask and then a diffraction grating 93b is formed.
According to the device shown in FIG. 7, the laser beam 17a output from the laser beam source 4 is divided into two beams by the half mirror 5 that are reflected by the mirrors 6a and 6b and meet again on the substrate 1. At this time, the intensity of the light on the substrate has a distribution with a period A because of the interference of the two beams. The period .LAMBDA. is represented by: EQU .LAMBDA.=(.lambda./(2 sin .theta.))
where .lambda. is the wavelength of the laser beam and .theta. is the incident angle of the laser beam on the substrate.
The conventional diffraction grating is formed using the above principle in which the resist applied to the substrate is exposed with interference fringes of light having a period .LAMBDA.. Then the resist is developed and the substrate is etched away using the patterned resist as a mask.
FIG. 9 is a sectional view showing a conventional single wavelength semiconductor laser device disclosed in Optics, Volume 15, Number 2, pages 115-121. In FIG. 9, an n-type InGaAsP guide layer 107, an InGaAsP active layer 108, and a p-type InP layer 109 are sequentially formed on an n-type InP substrate 101 in which a diffraction grating 102 having a central phase shift region is present. An n side electrode 110 is provided on a back surface of the substrate 101 and a p side electrode 111 is provided on the p-type InP layer 109. In addition, a non-reflective coating film 113 is provided on each end surface 114 of the laser.
In the semiconductor laser device, electrons in the n-type InP substrate 101 and holes in the p-type InP layer 109 are injected into the InGaAsP active layer 108 and then emissive recombination occurs. In the distributed feedback (DFB) laser device having the diffraction grating 102 with the phase shift region in an active region, the light generated by the emissive recombination is reflected by the diffraction grating 102 and oscillates within the laser element whereby laser light is produced.
Since the diffraction grating 102 effectively reflects light having a wavelength .lambda. where .lambda.=2n.sub.eff .LAMBDA./n (n.sub.eff is the equivalent refractive index, .LAMBDA. is the period of the diffraction grating, and n is an integer), the oscillation wavelength is the wavelength with the largest gain in the active region among wavelengths represented by 2 n.sub.eff .LAMBDA./n. As for the oscillation wavelength .lambda., when n=1, the diffraction grating is called a primary diffraction grating, and when n=2, the diffraction grating is called a secondary diffraction grating.
In the single wavelength oscillating semiconductor laser device having the structure shown in FIG. 9, since the light is reflected only by the diffraction grating 102 in the active region and then confined within the element, light intensity in the center of the element is increased. As a result, the linearity between light output and injected current and the stability of single wavelength oscillation are reduced by melting part of the laser.
As described above, according to the conventional method for manufacturing a diffraction grating, it is not possible to form a diffraction grating having a period which is less than one-half the wavelength .lambda. of the laser beam source 4.
In addition, according to the conventional single wavelength oscillating semiconductor laser, the linearity of the light output relative to injected current and the stability of the single wavelength oscillation is reduced by the melting part of the laser.